Prediction of metadynamic softening in a multi-pass hot deformed low alloy steel using artificial neural network

The metadynamic softening behaviors in 42CrMo steel were investigated by isothermal interrupted hot compression tests. Based on the experimental results, an efficient artificial neural network (ANN) model was developed to predict the flow stress and metadynamic softening fractions. The effects of deformation parameters on metadynamic softening behaviors in the hot deformed 42CrMo steel have been investigated by the experimental and predicted results from the developed ANN model. Results show that the effects of deformation parameters, such as strain rate and deformation temperature, on the softening fractions of metadynamic recrystallization are significant. However, the strain (beyond the peak strain) has little influence. A very good correlation between experimental and predicted results indicates that the excellent capability of the developed ANN model to predict the flow stress level and metadynamic softening, the metadynamic recrystallization behaviors were well evidenced.     

Constitutive descriptions for hot compressed 2124-T851 aluminum alloy over a wide range of temperature and strain rate

The compressive deformation behaviors of 2124-T851 aluminum alloy were investigated over a wide range of temperature and strain rate on Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit a peak stress at a very small strain, after which the flow stresses decrease until high strains, showing a dynamic flow softening. The measured flow stress was modified by friction correction, and the corrected flow stresses are lower than the measured ones, which nicely reflect negative effects of the interfacial friction on the flow stress. A revised model is proposed to describe the relationships of the flow stress, strain rate and temperature of 2124-T851 aluminum alloy at elevated temperatures. The stress–strain values of 2124-T851 aluminum alloy predicted by the proposed model well agree with experimental results, which confirmed that the revised deformation constitutive equation gives an accurate and precise estimate for the flow stress of 2124-T851 aluminum alloy.     

Effects of holding time and Zener-Hollomon parameters on deformation behavior of cast Mg-8Gd-3Y alloy during double-pass hot compression

Double-pass hot compression tests were carried out over a wide range of holding time (0–180?s) and Zener-Hollomon parameter (1.6E15–1.3E20) to study the deformation behavior of cast Mg-8Gd-3Y alloy. The flow curves show obvious work hardening and strain softening stages, leading to the peak stress of double-pass hot compression. Holding time and Zener-Hollomon parameter can significantly affect the second pass peak stress. It is found that increasing the holding time can cause a higher peak stress in the second pass deformation. The second pass stress reaches the peak stress of 71?MPa at Zener-Hollomom parameter of 1.6E15. When the parameter rises to 1.3E20, the second pass peak goes up to 237?MPa. In addition, the second pass peak stress is significantly higher than the unloading stress, which is opposite to the flow behavior of aluminum alloys. Residual stored deformation energy caused by the first pass deformation could be consumed by metadynamic recrystallization. Therefore, more strain energy is required for subsequent dynamic recrystallization, resulting in hardening behavior. A hardening fraction is defined to describe the deformation behavior quantitatively, which shows a positive correlation with the metadynamic recrystallization fraction. The metadynamic recrystallization leads to grain growth at the inter pass holding stage, diminishing dynamic recrystallization nucleation positions in the second pass deformation.     

Analysis of work hardening and recrystallization during the hot working of steel using a statistically based internal variable model

A mathematical model has been developed which describes the hot deformation and recrystallization behavior of austenite using a single internal variable: dislocation density. The dislocation density is incorporated into equations describing the rate of recovery and recrystallization. In each case no distinction is made between static and dynamic events, and the model is able to simulate multideformation processes. The model is statistically based and tracks individual populations of the dislocation density during the work-hardening and softening phases. After tuning using available data the model gave an accurate prediction of the stress–strain behavior and the static recrystallization kinetics for C–Mn steels. The model correctly predicted the sensitivity of the post deformation recrystallization behavior to process variables such as strain, strain rate and temperature, even though data for this were not explicitly incorporated in the tuning data set. In particular, the post dynamic recrystallization (generally termed metadynamic recrystallization) was shown to be largely independent of strain and temperature, but a strong function of strain rate, as observed in published experimental work.     

Deformation behavior and microstructure evolution in multistage hot working of TA15 titanium alloy: on the role of recrystallization

Interrupted compression tests of TA15 titanium alloy with initially equiaxed microstructure were carried out at deformation temperatures between 1173 to 1273 K and strain rates between 0.001 to 0.1 s⁻¹ to investigate the deformation behavior and microstructure evolution under multistage deformation. The TA15 alloy exhibits significant flow softening in both β and (α + β) working. It is found that the flow softening relates to dynamic recrystallization of β phases under current experimental conditions. In multistage β working, metadynamic recrystallization is the main softening mechanism during inter-pass holding. The grain refinement by metadynamic recrystallization leads to the decrease in peak stress upon reloading. In multistage (α + β) working, static recrystallization is the main softening mechanism during inter-pass holding. The static recrystallization kinetics increases with temperature and strain rate. The inter-pass holding has little influence on the morphology of the primary α phases. The β grain size is determined by spacing of primary α phases, which is more affected by working temperature but less dependent on strain rate and inter-pass holding time.     

Flow Behavior and Evolution of Microstructure during Hot Deformation for a High Mo Stainless Steel

1. IntroductionHigh Mo austenitic stainless steels are widelyapplied to oceanology, petroleum chemical industryef..[1'2], because of their good resistance to local corrosion and uniform corrosion. However, they arehard to process and normally have worse plasticity.Therefore, that are the main weaknesses of this kindof material. It is necessary to improve the processing properties during hot working and control themicrostructure. Some researches have been carriedout on austenitic stainless ste…     

Hot rolling simulations of austenitic stainless steel

The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.     

Metadynamic recrystallization of 300M steel after isothermal compression

Isothermal compression tests of 300M steel were performed on a Gleeble-1500 thermo-mechanical simulator at deformation temperatures ranging from 1173 to 1373 K, strain rates ranging from 0.1 to 5.0 s^?1, and a strain of 0.69. Metadynamic recrystallization and grain growth after complete metadynamic recrystallization were investigated by isothermal compression with different inter-pass times. It was found that the inter-pass time, deformation temperature and strain rate markedly affected the austenite grains size of metadynamic recrystallization. The austenite grain size model and grain growth model of metadynamic recrystallization were determined based on the results of quantitative grain size. A good agreement between the predicted and measured austenite grain size and grain growth of metadynamic recrystallization was obtained, and the present models were effective to predict the austenite grain size and grain growth of metadynamic recrystallization in the isothermal compression of 300M steel.     

Deformation behavior and constitutive model for high temperature compression of a newly type of Ni3Al‐based superalloy

In order to investigate the hot deformation mechanism of a newly development Ni₃Al‐based superalloy, hot compression tests at temperatures between 1100 °C–1250 °C and the strain rates of 0.001 s^?1–1.0 s^?1 were conducted. The results show that the curves of true stress‐strain indicate the thermal deformation is a typical dynamic recrystallization process, which the peak stresses and steady‐state stresses increase with decreasing temperatures and increasing strain rates. The softening mechanism is mainly dynamic recrystallization. The experimental data of peak stresses and steady‐state stresses is employed to calculate the constants in the Arrhenius equation. The steady‐state stresses are considered more reasonable for solving the parameters in the Arrhenius equation. Based on the constitutive equation obtained, the calculated values of steady‐state stresses match well with the experimental values at the strain rates of 0.001 s^?1, 0.01 s^?1 and 0.1 s^?1, whereas there exists much deviation at 1.0 s^?1. For the sake of accuracy of predicted results at 1.0 s^?1 strain rate, a modified Zener‐Hollomon parameter Z’ is introduced. The results show that the modified constitutive equations established in this study could well predict the value of steady‐state stress in hot deformation of the newly development Ni₃Al‐based alloy.     

Modeling high-temperature tensile deformation behavior of AZ31B magnesium alloy considering strain effects

The hot tensile deformation behaviors of AZ31B magnesium alloy are investigated over wide ranges of forming temperature and strain rate. Considering the effects of strain on material constants, a comprehensive constitutive model is applied to describe the relationships of flow stress, strain rate and forming temperature for AZ31B magnesium alloy. The results show that: (1) The effects of forming temperature and strain rate on the flow behaviors of AZ31B magnesium alloy are significant. The true stress–true strain curves exhibit a peak stress at small strains, after which the flow stress decreases until large strain, showing an obvious dynamic softening behavior. A considerable strain hardening stage with a uniform macroscopic deformation appears under the temperatures of 523 and 573 K. The strain hardening exponent (n) increases with the increase of strain rate or the decrease of forming temperature. There are not obvious strain-hardening stages when the forming temperature is relatively high, which indicates that the dynamic recrystallization (DRX) occurs under the high forming temperature, and the balance of strain hardening and DRX softening is easy to obtain. (2) The predicted stress–strain values by the established model well agree with experimental results, which confirm that the established constitutive equation can give an accurate and precise estimate of the flow stress for AZ31B magnesium alloy.     

Constitutive modeling for the hot deformation behavior of Mg–3.06Zn–0.58Zr–1.07Y alloy

In order to improve the understanding flow behaviors of hot compressive deformation as‐homogenized of Mg–3.06Zn–0.58Zr–1.07Y alloy, carried out a series of isothermal compressive tests with 60% height reduction of specimens were performed at constant temperature of 523 K, 573 K, 623 K, 673 K, and 723 K, and strain rates of 0.001, 0.01, 0.1, and 1 s^?1 on Gleeble‐1500 thermo‐mechanical simulator. The results of the true stress–strain curves show that the flow stress increases with the increasing strain rate and decreasing deformation temperature. The flow behavior at constant strain rate was characterized by the dynamic recrystallization and dynamic recovery softening mechanisms occur simultaneously. The number of the dynamic recrystallization curve increases with increasing strain rate. A nonlinear flow model and its constitutive equation, based on the Arrhenius‐type equation, were employed for studying the deformation behavior and relationships between the deformation temperature, strain rate, and flow stress. Finally, the processing map of Mg–3.06Zn–0.58Zr–1.07Y alloy at the strain of 0.3 was obtained through the dynamic materials modeling. The optimal processing temperature and strain rate, using the microstructure and constitutive modeling, were found to be in the rage 623–723 K and 0.1–1 s^?1, respectively.     

Effect of processing parameters on the hot deformation behavior of as-cast TC21 titanium alloy

Isothermal compression tests of as-cast Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb (TC21) titanium alloy are conducted in the deformation temperature ranging from 1000 to 1150 °C with an interval of 50 °C, strain rate ranging from 0.01 to 10.0 s⁻¹ and height reductions of 30%, 45%, 60% and 75% on a computer controlled Gleeble 3500 simulator. The true stress–strain curves under different deformation conditions are obtained. Based on the experimental data, the effects of deformation parameters on the hot deformation behavior of as-cast TC21 alloy were studied. The deformation mechanisms of the alloy in the whole regimes are predicted by the power dissipation efficiency and instability parameter and further investigated through the microstructure observation. It is found that at the height reductions of 30%, 45% and 60%, the softening of stress–strain curves at high strain rate (>1.0 s⁻¹) is mainly associated with flow localization, which is caused by local temperature rise, whereas at low strain rate, the softening is associated with dynamic recrystallization (DRX). However, the instability showed in flow localization occurs at low strain rate of 0.01 s⁻¹ when the height reduction reaches 75%. In addition, the effects of strain rate, deformation temperature and height reduction on microstructure evolution are discussed in detail, respectively.     

Dynamic recrystallization behavior of an as-cast TiAl alloy during hot compression

High temperature compressive deformation behaviors of as-cast Ti–43Al–4Nb–1.4W–0.6B alloy were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s^− 1 to 1 s^− 1. Electron back scattered diffraction technique, scanning electron microscopy and transmission electron microscopy were employed to investigate the microstructural evolutions and nucleation mechanisms of the dynamic recrystallization. The results indicated that the true stress–true strain curves show a dynamic flow softening behavior. The dependence of the peak stress on the deformation temperature and the strain rate can well be expressed by a hyperbolic-sine type equation. The activation energy decreases with increasing the strain. The size of the dynamically recrystallized β grains decreases with increasing the value of the Zener–Hollomon parameter (Z). When the flow stress reaches a steady state, the size of β grains almost remains constant with increasing the deformation strain. The continuous dynamic recrystallization plays a dominant role in the deformation. In order to characterize the evolution of dynamic recrystallization volume fraction, the dynamic recrystallization kinetics was studied by Avrami-type equation. Besides, the role of β phase and the softening mechanism during the hot deformation was also discussed in details.     

Dynamic and metadynamic recrystallization of Hastelloy X superalloy

Hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹ were performed on Hastelloy X superalloy in order to investigate the kinetics of hot deformation. An Arrhenius-type equation was used to characterize the dependence of the flow stress on deformation temperature and strain rate. The results showed that dynamic recrystallization (DRX) as well as metadynamic recrystallization (MDRX) occurred during hot working. A novel technique has been developed for calculating the DRX kinetics parameters on the basis of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isothermal transformation rate equations. The variation of grain size in the DRX and MDRX regimes correlated with the standard Zener–Hollomon parameter.     

Mn18Cr18N护环钢静态再结晶组织及模型

在Gleeble-1500D热模拟机上,采用双道次热压缩试验研究Mn18Cr18N护环钢高温变形后不同停留时间内的静态软化行为,分析热变形温度、应变速率、变形程度以及初始奥氏体晶粒对静态再结晶行为的影响.通过应力补偿法计算静态再结晶软化率,并结合金相组织作了修正.建立其静态再结晶动力学模型,获得静态再结晶激活能249.3 k J/mol.研究表明:Mn18Cr18N钢静态再结晶软化曲线呈"S"形,符合Avrami方程.静态再结晶体积分数随着停留时间延长而增加,热变形温度越高,静态再结晶分数越大,而在较低温度和较小变形程度时,孕育时间较长,主要发生静态回复,将静态再结晶动力学模型的预测结果与实测值进行比较,二者吻合较好,为护环钢后续热镦粗工艺模拟提供更为详尽的模型.     

5083铝合金的热变形组织演变及晶粒度模型

通过等温压缩实验、光学显微镜与透射电镜研究了变形温度300~450℃、应变速率0.01~1s-1、真应变0.36~1.2范围内变形条件对5083铝合金热变形组织演变的影响。结果表明:升高热变形温度或降低应变速率均可促进5083铝合金的动态再结晶发生,使变形后5083铝合金位错密度降低,再结晶晶粒尺寸增大;随着应变量的增加,变形后合金的位错密度降低,动态再结晶程度增大。根据唯象理论的指数模型,利用线性回归方法建立了5083铝合金动态再结晶晶粒度模型,模型计算值与实测值吻合良好,平均相对误差仅为4.6%。     

Constitutive modeling for flow stress of 55SiMnMo bainite steel at hot working conditions

The hot deformation behavior of 55SiMnMo bainite steel was studied through isothermal hot compression tests conducted using a Gleeble 3500 at 950–1100 °C, with strain rates of 0.01 s⁻¹ to 10 s⁻¹. A constitutive equation was established using the experimental results to describe the stress–strain relationship based on the dislocation density variation, considering the influence of the dynamic softening mechanism. When dynamic recovery is the only softening mechanism, a constitutive equation for flow stress was obtained from the variation of the dislocation density during hot deformation based on work hardening and dynamic recovery. When dynamic recrystallization occurs, the relationship between the dislocation density and the volume fraction of dynamic recrystallization was used to predict the flow stress after the peak. The reliability of the model was verified through a comparison between the predicted flow stress curves from the model and the experimental data.     

Hot tensile deformation behaviors and constitutive model of 42CrMo steel

The hot tensile deformation behaviors of 42CrMo steel are studied by uniaxial tensile tests with the temperature range of 850–1100 °C and strain rate range of 0.1–0.0001 s⁻¹. The effects of hot forming process parameters (strain rate and deformation temperature) on the elongation to fracture, strain rate sensitivity and fracture characteristics are analyzed. The constitutive equation is established to predict the peak stress under elevated temperatures. It is found that the flow stress firstly increases to a peak value and then decreases, showing a dynamic flow softening. This is mainly attributed to the dynamic recrystallization and material damage during the hot tensile deformation. The deformation temperature corresponding to the maximum elongation to fracture increases with the increase of strain rate within the studied strain rate range. Under the strain rate range of 0.1–0.001 s⁻¹, the localized necking causes the final fracture of specimens. While when the strain rate is 0.0001 s⁻¹, the gage segment of specimens maintains the uniform macroscopic deformation. The damage degree induced by cavities becomes more and more serious with the increase of the deformation temperature. Additionally, the peak stresses predicted by the proposed model well agree with the measured results.     

2124铝合金的热压缩变形和加工图

采用热模拟实验研究2124铝合金在应变速率为0.01~10s-1、变形温度为340~500℃条件下的流变应力行为。结果表明:2124铝合金热变形过程中的流变应力可用双曲正弦本构关系来描述,平均激活能为170.13kJ/mol。根据动态材料模型,计算并分析2124铝合金的加工图。利用加工图确定热变形的流变失稳区,并且获得了实验参数范围内的热变形过程的最佳工艺参数,其热加工温度为450℃左右,应变速率为0.01~0.1s-1。     

新型Al-Zn-Mg-Sc-Er-Zr合金的热变形行为EI北大核心CSCD

采用Gleeble-3800热模拟机研究Al-8.9Zn-1.3Mg-0.1Sc-0.1Er-0.1Zr铝合金的热变形行为,构建温度380~440℃、应变速率0.01~10 s^(-1)区间内合金的热加工图,使用X射线衍射(XRD)、选区电子衍射(SAED)与能谱(EDS)对合金中存在的物相进行分析,并使用金相显微镜(OM)和透射电子显微镜(TEM)观察合金热变形后的微观组织。结果表明:合金的最佳热加工工艺参数区间为:400℃相似文献